Band pass amplifier



2 Shets-Sheet 1 Filed April 24, 1937 Capacitive at rejection frequen cies I 3m2entor -Gu-stave Grundmann (Ittorneg Patented July 16, 1940 UNITED STATE'S PATENT ol-"rics; A

BAND PASS AMZPLIFIER Gustave L. Grundmann, Westmont, N. .I., assignor to Radio Corporation of America, a corpora- ,tion of Delaware Application April 24, 1931, Serial No. 138,684

11 .Claims.

In television systems it is the preferred prac-.

10 tice to transmit the picture signals on one carrier wave and to transmit the sound signals on an adjacent carrier wave, the two carrier waves being as closely spaced as the problem of obtaining good separation of the picture and sound signals 16 at the receiver will permit.

In order to prevent the sound signals from entering the picture signal channel, it has been the practice to employ some kind of rejector circuit associated withv the picture channel. In general,

go however, it has been found difilcult to obtain a satisfactory circuit which sufllciently attenuates the sound signals.

It is, accordingly, an object of my invention to provide an improved intermediate frequency am- 25 plifier which greatly'attenuates signals lying outside but ,close to the amplifier pass range.

It is a further object of my invention to provide an improved intermediate frequency amplifier for a combined picture and sound receiver 30 which has a wide pass band for picture signals and which has a sharp cut-,ofl characteristic on the side of the pass band adjacent to the sound signals.

It is a further object of my invention to provide 35 an improved band pass filter having a sharp cutoif on at least one side of its band pass range.

It is a still further object of my invention-to provide an improved band pass amplifier having a sharp cut-off, and in which voltage may be applied to the plate of an amplifier tube through an inductance coil.

In the preferred embodiment of my invention,

I employ-two coupled tuned circuits, each circuit being tuned substantially to the midpointof the desired pass range and the circuits being coupled and damped the correct amount to give an approximately fiat-topped selectivity curve. It will be understood that the coupled circuits may be tuned to slightly different frequencies to make 50 the selectivity curve symmetrical in accordance with common practice. The desired coupling is obtained by means of an inductance coil common to the two circuits. In combination with the band pass filter thus formed, I employ an atu tenuation network which, either in its physical form or in its equivalent form, is a bridged-T network. The bridged-T network is adjusted to reject a narrow frequency band just outside the pass band of the filter, this narrow frequency band being'the sound signals in the case of a television receiver.

The invention will be better understood from the following description, taken in connection with the accompanying drawings, in which Figures 1, 2 and 3 are circuit diagrams of several embodiments of my invention,

Figure 4 is an equivalent circuit diagram for the circuit shown in Fig. 3,

Figure 5 is a diagram showing the physical arrangement of the coils shown in Fig. 3, and

Figure 6 is a curve showing the characteristic of the filter shown in Fig. 3.

Referring to Fig. 1,'there is shown an intermediate frequency amplifier stage designed in accordance with my invention, for use 'in a tele- 0 vision receiver. This amplifier stage comprises an amplifier tube I, which maybe of the usual screen grid type having a cathode 2, a control grid 3, a screen grid 4, and a plate 8. The output electrodes of tube I are coupledto the input electrodes of the amplifier tube I, in the succeeding amplifier stage, through my improved filter circult. The tube 1 may be of the screen grid type having a cathode 8, a control grid 9, a screen grid II, and a plate I2.

My improved filter circuit includes a tuned primary circuit comprising a primary coil I3 which is tuned by a capacitor It in parallel with the output capacity of tube I, or by the tube capacity alone, and a tuned secondary circuit comprising a secondary coil I 8 which is tuned by the capacitor I1 iifparallel with the input capacity of tube I, or by the tube input capacity alone. The two tuned circuits are coupled by an inductance coil I8 which is common to the primary and secondary circuits. In order to provide the necessary damping for broad tuning, a resistor I5 may be connected across the coil I3.

The condensers III and 20 are filter and blocking condensers, respectively, while the resistor 25 is a grid resistor through which bias is applied to grid 9.

In accordance with this particular embodiment of my invention, a balanced bridged-T rejector network 2| .is connected in either the primary circuit or the secondary circuit, it being connected in the primary circuit in the amplifier illustrated. The network 2i comprises an inductance coil 22 connected in series with primary coil I3 and coupling coil II, and two capacitors in parallel to the coil 22. The capacitors 23 and 24 are of: like capacity in the case of a symmetrical networkl The rejector network further comprises a resistor 26 connected between ground and the junction point of capacitors 23 and 24. By properly adjusting the resistor 26 the effect of the unavoidable resistance in coil 22 can be cancelled.

In the amplifier being described, it is desired to pass a wide frequency band of picture signals and to reject a narrow frequency band of sound signals on the low frequency side of the picture signals. To eflect this rejection, the rejector circuit 22, 23, 24-is tuned to the frequency of the sound carrier wave. The resistor 26 is then adjusted until no signal at this frequency appears between the point A and ground. This result can be obtained because, as will be described hereinafter, the resistor 26 can be made to effectively balance out the resistance of the coil 22.

Since the rejector circuit is resonant at a frequency below the picture frequencies, that is, below the pass range of the filter, it presents a capacitive reactance within this pass range. Therefore, within the pass range, there is in series with the primary tuning capacity a capacity presented by the rejector circuit. It follows that if the primary coil I3 has previously been adjusted for proper tuning without the rejector circuit, it must be readjusted when the rejector circuit is added to again tune the primary circuit to the mid-point of the pass band.

At this point it may be helpful to trace the primary and secondary circuits of my filter. The.

primary circuit may be traced from the lower end of coupling coil i3, through the coupling coil I8, through the capacity presented by the parallel resonant circuit 22, 23, 24, through the primary coil I3, through the capacity presented by the tube and capacitor i4 to ground, and through the filter or by-pass condenser back to the lower end of coil la.

The secondary circuit may be traced from the lower end of coupling coil l3, through blocking condenser 20 and secondary coil l6, through the capacity presented by tube 1 and capacitor i! to ground, and through the filter condenser l3 back to the lower end of coil Hi.

It may be noted that the inductance coils i3, 22 and i6 preferably are made variable by any suitable means such as movable magnetic'cores or the like.

By means of the above described circuit, it is possible to obtain an exceedingly sharp cut-oil on one side of the filter pass rangeas indicated by the curve shown in Fig. 6. This specific curve is for a circuit which will be described later, but the circuit just described has the same general characteristics.

Referring again to the symmetrical rejecto circuit 2|, the resistance R of resistor 26 which will give zero voltage between the point A and ground at a certain frequency, is determined by the following equation:

where w is 21r times the frequency of the signal to be rejected, 0 is the capacity of each of the condensers 23 and 24, and r is the resistance of coil 22.

The rejector network may be unsymmetrical if desired, that is, condensers 23 and 24 may have different capacities. For the unsymmetrical case,

the value of R cannot be expressed by the equation given above, but it is a lower value than for the symmetrical case and can readily be determined by varying the value of resistor 23.

If it is desired to reject a signal on the high frequency side of the filter pass band, the rejector is tuned to that frequency. The circuit 22, 23, 24 then presents an inductive reactance throughout the pass range of the filter, and the inductance of primary coil l3 must be decreased in value a corresponding amount, assuming it has previously been adjusted without the rejector circuit included in the filter.

In Fig. 2 I have shown a circuit which is the same as shown in Fig. 1, except that a different bridged-T network is-employed. In the two figures. like parts are indicated by the same reference numerals.

In Fig. 2 the rejector circuit comprises an inductance coll 3| in series with the primary coil 3 and the coupling coil i3, the coil 3| being bridged by a variable capacitor 32. A resistor 33 is connected between the mid-point of coil 3| and a point which is at ground potential at the frequency of the signal to be rejected. This rejector .circuit is adjusted the same as the rejector circuit 2|, the circuit 3|, 32 being tuned to the signal to be rejected, and the resistor 33 then being adjusted until none of this signal appears on the output side of the circuit. a

' It will be noted that in both of the above described filter circuits the plate voltage for amplifier tube is supplied through inductance coils rather than through aresistor, whereby it is unnecessary to supply the voltage drop in a resistor in addition to the voltage required by the tube.

In Fig. 3 there is shown another embodiment of my invention which has been found to give excellent rejection of sound signals in a television receiver. In Figs. 1 and 3 like parts are indicated by the-same reference numerals.

In Fig. 3, as in the other figures, the band pass portion of the filter comprises two coupled tuned circuits, the coupling being provided by the coil l3 common to the 'two circuits. The rejector portion of the circuit, however, differs considerably from the previously described bridged-T networks in physical construction, although, as will be explained in connection with Fig. 4, it is similar electrically to the bridged-T network shown in Fig. 1.

The rejector portion of the circuit in Fig. 3

comprises an inductance coil 4| in series with the primary coil l3 and coupling coil IS, the coil 4| being inductively coupled to coil l3 but not to coil I8. This inductive coupling is in such direction as to provide a negative mutual inductance, which, as is well known, is a capacity reactance.

The coil 4| is bridged by an inductance coil 42 and a capacitor 43 connected in series. The coil 42 preferably is provided with a variable magnetic core or the like for varying its induct- In this particular circuit is also provided the necessary damping for the two coupled tuned circuits.

For the purpose of illustration, it will be assumed that the circuit oi Fig. 3 is to reject the sound signal on the low frequency side of the piclines.

coupled primary and secondary circuits are eachtuned to the approximate mid-point or mid-frequency of the picture signal pass band by adjusting coils l3 and I6. Finally, the resistance of resistor 44 is varied until no sound signal ap pears between the point A and ground.

The nature of the circuit of Fig. 3 will be better understood by referring-to the equivalent circuit diagram in Fig. 4. It will be seen from this diagram that the negative mutual inductance indicated at --M is effectively in series with the coil 42 and the capacitor 43 to bridge the coil, and that the resistor 44 is effectively connected between ground and the junction point of -M and coil 42.

It has previously been mentioned that at one frequency, here the sound signal frequency, the negative inductance M is equivalent to a capacity. The elements 42, 43 in series also present a capacitive reactance at the rejection frequency, the value of -M not being great enough to tune the coil 4|. Thus it is apparent that the equivalent-rejector circuit in Fig. 4 is of the same type as shown in Fig. 1. In the circuit illustrated, however, it is unsymmetrical.

Fig. 5 shows the relative locations of the several coils in the circuit illustrated in Fig. 3, like coils in the two figures being indicated by the same reference numerals. The coils l3, 4| and 42 are wound on one supporting tube while the coils l6 and iii are wound on another supporting tube. Shielding preferably is provided between the two groups of coils as indicated by the dotted Each supporting tube, in the specific circuit shown, has an outside diameter of it; inch, the coils being wound on the outside of the tubes and having the number of turns indicated. All coils are single layer close wound and of .008 inch enamel covered wire. These specific values are given to indicate the order of magnitude of the various inductance values.

The several capacities which determine the tuning have been indicated in micro-microfarads for this specific circuit. The value of resistor 44 when adjusted for good rejection is indicated in ohms.

As indicated on the drawing in dotted lines, the tuning capacity of the secondary circuit is provided solely by the input capacity 41 of tube 1, while the tuning capacity of the primary circuit is provided by the output capacity of tube indicated'at 48 and a small capacitor 49 in parallel therewith.

The selectivity curve of the circuit of Fig. 3 having the particular constants given above is shown in Fig. 6. It will be seen that the circuit passes the picture frequencies lying between 9.1

megacycles and 11 megacycles substantially without attenuation and that it rejects the sound signals having the carrier frequency of 8.75 megacycles.

It is to be understood that specific details and values have been given in describing the circuit of Fig. 3 merely to give a complete disclosure of the invention as required by the statutes and that the invention is not limited to such construction and values. I have given specific values for the several elements in this circuit largely because the best value for the resistor 44 cannot be expressed conveniently in an equation.

Although I have shown the rejector circuit in the primary circuit in the several figures, it

should be understood that it may be located in the secondary circuit instead. Also, the rejector network may be located on either side of the primary or secondary'coil. For example, in Fig. 3 the rejector network may be located between the tube I and the coil l3 if preferred.

I claim as my invention:

1. A band pass filter comprising a pair of circuits each tuned to approximately the same frequency and so coupled a d damped as to have a broad-topped selectivi y curve whereby said filter has a certain pass band, and a rejector network in one of said tuned circuits, said rejector network including a parallel resonant circuit tuned to a frequency which is outside of but close to said pass band and also including means for balancing out the cifect of resistance in said parallel resonant circuit whereby complete rejection of a signal at said last-mentioned frequency is effected.

2. The invention according to claim 1 characterized in that the said pair of circuits are coupled through an inductance coil common to said pair of circuits.

3. A band pass filter comprising a pair of circuits each tuned to approximately the'same frequency and so coupled as to have a broad-topped selectivity curve whereby said filter. has a certain pass band, and a bridged-T rejector network in one of said tuned circuits, said rejector network including a parallel resonant circuit tuned to a signal having a frequency which is outside of but close to said pass band and a resister connected between a point which is at ground potential at said frequency and a tap on said parallel resonant circuit.

4. The invention according to claim 3 characterized in that the said pair of circuits are coupled through an inductance coil common to said pair of circuits.

5. A band pass filter comprising a pair of circuits each tuned to substantially the same frequency,

one circuit including a. primary tuning coil and the other circuit including a secondary tuningcoil, a coupling coil common to said pair of circuits, said coupling coil having such value as to give said filter a broad-topped characteristic whereby it has a certain pass band, and a bridged-T rejector network in one of said circuits, said network comprising a parallel resonant circuit in the high potential side of said one circuit and in series with the tuning coil of said one circuit and said coupling coil and further comprising a resistor connected between the low potential side of said one circuit and. a tap on said parallel resonant circuit, said parallel resonant circuit being tuned to a frequency which lies outside of but close to said pass band.

6. A band pass amplifier comprising an amplifier having output electrodes including an anode, a second amplifier having input electrodes. a band pass filter coupling said output electrodes to said input electrodes, said band pass filter comprising a pair of coupled tuned circuits, said circuits being coupled through an inductance coil common to said tuned circuits, a bridged-T rejector network in one of said tuned circuits. said network comprising an inductance coil having a capacitor connected thereaeross to form a parallel resonant circuit which is tuned to the frequency of a signal to be rejected, means for supplying an operating voltage to said anode through said coupling coil and said network inductance coil in series, and a resistor connected between ground and a tap on said parallel resonant circuit. v

7. A band pass amplifier comprising an amplifier tube having output electrodes, a second a'mplifier tube having input" electrodes, means for coupling said output electrodes and said input electrodes, said means comprising a tuned primary circuit including a primary coil and a coupling coil, a tuned secondary circuit also including said coupling coil, and a bridged-T network included in said tuned primary circuit, said network comprising a rejector coil in series with said primary coil and said coupling coil, said rejector coilbeing bridged by at least one capacitor and being tuned thereby tqthe frequency of the signal to be rejected to form a resonant circuit, anda resistor connected between a point which is at ground potential at said frequency and a point on said resonant circuit intermediate the ends of said rejector coil, said primary and secondary circuits each being tuned to the mid-point of the frequency band to be passed thereby and being so coupled as to give a substantially fiat-topped selectivity curve.

8; A band pass filter comprising a pair of coupled tuned circuits each including an inductance'kzoil, a rejector circuit connected in one of said tuned circuits, said rejector circuit comprising a rejector coil in series with and adjacent to the inductance coil of said one tuned circuit, said last two coils .being so coupled as to have a negative mutual inductance, a second rejector coil and a capacitor in series with each other and connected across the first rejector coil to form a parallel resonant circuit, said parallel resonant circuit being tuned to the frequency of the signal to be rejected, and aresistor connected between a point which is at ground potential at said signal frequency and the junction point of said first rejector coil and the inductance coil of said one circuit.

9. A band pass amplifier comprising an amplifier tube having output electrodes including an anode and a second amplifier having input electrodes, a band pass filter coupling said output electrodes and said input electrodes, said filter comprising a tuned primary circuit having a tuning coil and a coupling coil in series and a tuned secondary circuit having a tuning coil in series with said coupling coil, a rejector circuit in one of said tuned circuits and comprising an inductance coil in series with the tuning coil of said one circuit and adjacent thereto, said last two coils being so coupled as to have a negative mutual inductance, a second inductance coil and a capacitor being connected cuit in series with said primary in series with each other'and across the first an anode and a second amplifier having input electrodes, a band pass filter coupling said out-z put electrodes and said input e.ectrodes, said filter comprising a tuned primary circuit having a primary coil and a coupling coil in series and a tuned secondary circuit having a secondary coil in series with said coupling coil, a

rejector circuit comprising an inductance coll in series with said primary coil and adjacent thereto, said last two coils being so coupled as to have a negative mutual inductance, a second inductance coil and a capacitor being connected in series with each other and across the first inductance coil to form a parallel resonant ciroil and said coupling coll, said parallel resona t circuit being tuned to the frequency of thevsignal to be rejected, a resistor connected between a point at ground potential at said frequency and the junction point of said primary coil and said fli'st inductance coil, and means for supplying an operating voltage ,7 to said anode through sa'ld coupling coilisaid primary coil and said first inductance coil in series.

11. A band pass filter comprising-a pair of coupled tuned circuits, one of said clrcuits'being a primary circuit including an inductance coil, a rejector circuit connected in said primary circuit, said rejector circuit comprising a rejector coil in series with and adjacent to said inductance soil, said two coils being so coupled as to have a negative mutual inductance, a second rejector coil and a capacitor in series with each other and connected across the first rejector coil to form a parallel'resbriant circuit, said parallel resonant circuit being tuned to the frequency of thesignal to be rejected, and a resistor connected between a point which is at ground potential at said signal frequency and the junction point of said first rejector coil and said inductance coil,

GUSTAVE L. GRUNDMANN. 

